Conversion of La2_2Ti2_2O7_7 to LaTiO2_2N via Ammonolysis: An ab-initio Investigation

Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A$_2$B$_2$O$_7$}) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH$_3$ decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70% of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride

Synthesis and Structure of the Double-Layered Sillén-Aurivillius Perovskite Oxychloride La2.1Bi2.9Ti2O11Cl as a Potential Photocatalyst for Stable Visible Light Solar Water Splitting.

Exploring photocatalysts for solar water splitting is a relevant step toward sustainable hydrogen production. Sillén-Aurivillius-type compounds have proven to be a promising material class for photocatalytic and photoelectrochemical water splitting with the advantage of visible light activity coupled to enhanced stability because of their unique electronic structure. Especially, double- and multilayered Sillén-Aurivillius compounds [An-1BnO3n+1][Bi2O2]2Xm, with A and B being cations and X a halogen anion, offer a great variety in material composition and properties. Yet, research in this field is limited to only a few compounds, all of them containing mainly Ta5+ or Nb5+ as cations. This work takes advantage of the outstanding properties of Ti4+ demonstrated in the context of photocatalytic water splitting. A fully titanium-based oxychloride, La2.1Bi2.9Ti2O11Cl, with a double-layered Sillén-Aurivillius intergrowth structure is fabricated via a facile one-step solid-state synthesis. A detailed crystal structure analysis is performed via powder X-ray diffraction and correlated to density functional theory calculations, providing a detailed understanding of the site occupancies in the unit cell. The chemical composition and the morphology are studied using scanning and transmission electron microscopy together with energy-dispersive X-ray analysis. The capability of the compound to absorb visible light is demonstrated by UV-vis spectroscopy and analyzed by electronic structure calculations. The activity toward the hydrogen and the oxygen evolution reaction is evaluated by measuring anodic and cathodic photocurrent densities, oxygen evolution rates, and incident-current-to-photon efficiencies. Thanks to the incorporation of Ti4+, this Sillén-Aurivillius-type compound enables best-in-class photoelectrochemical water splitting performance at the oxygen evolution side under visible light irradiation. Thus, this work highlights the potential of Ti-containing Sillén-Aurivillius-type compounds as stable photocatalysts for visible light-driven solar water splitting

Linking Morphology and Multi-Physical Transport in Structured Photoelectrodes

Semiconductors with complex anisotropic morphologies in solar to chemical energy conversion devices enhance light absorption and overcome limiting charge transport in the solid. However, structuring the solid-liquid interface has also implications on concentration distributions and diffusive charge transport in the electrolyte. Quantifying the link between morphology and those multi-physical transport processes remains a challenge. Here we develop a coupled experimental-numerical approach to digitalize the photoelectrodes by high resolution FIB-SEM tomography, quantitatively characterize their morphologies and calculate multi-physical transport processes on the exact geometries. We demonstrate the extraction of the specific surface, shape, orientation and dimension of the building blocks and the multi-scale pore features from the digital model. Local current densities at the solid-liquid interface and ion concentration distributions in the electrolyte have been computed by direct pore-level simulations. We have identified morphology-dependent parameters to link the incident-light-to-charge-transfer-rate-conversion to the material bulk properties. In the case of a structured lanthanum titanium oxynitride photoelectrode (Eg = 2.1 eV), with an absorptance of 77%, morphology-induced mass transport performance limitations have been found for low bulk ion concentrations and diffusion coefficients

Determination and optimization of material parameters of particle-based LaTiO2N photoelectrodes

We developed a validated numerical model capable of predicting the photocurrent-voltage characteristics of oxide and oxynitride particle-based photoelectrodes and identifying the critical parameters affecting the performance of those photoelectrodes. We used particle-based LaTiO2N photoelectrodes as the model system. Two different types of electrodes were studied: LaTiO2N photoelectrodes with TiO2 inter-particle connections and the same photoelectrodes with NiOx/CoOx/Co(OH)2 co-catalysts and a Ta2O5 passivation layer. The necessary material parameters, namely complex refractive index, permittivity, density of states of the conduction and valence bands, charge mobilities, flatband potential, doping concentration, recombination lifetimes, and interfacial hole transfer velocity, were derived by density functional theory calculations, dedicated experiments, and fitting of the numerically determined photocurrent-voltage curves to the measured ones under back-side illumination. The model was validated by comparing its prediction to front-side illumination photocurrent-voltage measurements. A parametric study was then carried out to provide an extensive set of material design guidelines and key parameters for high-performing particle-based LaTiO2N photoelectrodes. The interfacial hole transfer velocity was identified as the most significant parameter for the performance of LaTiO2N photoelectrodes

Scaling Up electrodes for photoelectrochemical water splitting : fabrication process and performance of 40cm2 LaTiO2N photoanodes

A scalable process for fabrication of particlebased photoanodes is developed. The electrodes are versatilely made of photocatalytically active semiconductor particles, in this case LaTiO2N, and optionally coated with cocatalysts and protecting components, all immobilized on a conducting substrate. The involved fabrication steps are restricted to scalable processes such as electrophoretic deposition, annealing in air, and dip coating. Special care is taken to ensure efficient charge transport inbetween particles and to the substrate by incorporating conducting connectors. By adapting the fabrication steps, the electrode geometrical dimension is increased from the size of a typical lab electrode of 1 to 40cm2. The quality of the scaleup process is characterized by comparing the photoanodes in terms of thickness, lightabsorption properties, and morphology. For several compositions, the electrochemical performance of both electrode sizes is assessed by measuring the photocurrents and faradaic efficiencies. The comparison revealed a complex upscaling behavior and showed that the photoelectrode size affects performance already on the 0.1m scale.(VLID)460373

Majority Charge Carrier Transport in Particle-based Photoelectrodes

The inter-particle charge transfer of particle-based photoelectrodes was investigated using a particle-based LaTiO2N photoelectrode as model system. The thickness-dependent front- to back-side illumination photocurrent ratio was measured and compared to the numerical photogenerated current ratio. This comparison suggested the presence of majority charge carrier transport limitations and estimated that only a particle-based film thickness of 450 nm was contributing to the photocurrent. We introduced three different theoretical inter-particle charge transfer mechanisms and implemented their respective equations in a numerical model. The calculated photocurrent-voltage curves were compared to experimental data and proved that inter-particle charge transfer is negligible. Only the particles in direct contact with the fluorine doped tin oxide glass substrate were contributing to the photocurrent. Thus, more efficient particle-based photoelectrodes should incorporate efficient conductive networks connecting particles and substrate. The simulations indicate that the photocurrent density of particle-based photoelectrodes could be increased from 1.2 mA cm-2 to 5 mA cm-2 at 1.23 VRHE under front-side illumination when adding such a conductive network between particles and substrate